The basic concept of a Stirling engine dates back to a patent registered by Robert Stirling in 1817. Since that time, this engine has been the subject of intense scrutiny and evaluation. Various Stirling engine systems have been prototyped and put into limited operation throughout the world. One potential application area for Stirling engines is for automobiles as a prime mover or engine power unit for hybrid electric applications. Other fields of potential use of a Stirling engine such as stationary auxiliary power units, marine applications and solar energy conversion.
Stirling engines have a reversible thermodynamic cycle and therefore can be used as a means of delivering mechanical output energy from a source of heat, or acting as a heat pump through the application of mechanical input energy. Using various heat sources such as combusted fossil fuels or biogases, or concentrated solar energy, mechanical energy can be delivered by the engine. This energy can be used to generate electricity or can be directly mechanically coupled to a load.
The Assignee of the present application, Stirling Biopower, Inc. and its predecessor company have made significant advances in the technology of Stirling machines through a number of years. Although the Assignee has achieved significant advances in Stirling machine design, there is a constant need to further refine the machine, particularly if the intended application is in large volume production.
The Stirling engine of the present invention bears many similarities to those previously developed by Assignee and its predecessor company, including those described in U.S. Pat. Nos. 4,439,169; 4,481,771; 4,532,855; 4,579,046; 4,615,261; 4,669,736; 4,836,094; 4,885,980; 4,707,990; 4,996,841 4,977,742; 4,994,004; and 5,074,114, which are hereby incorporated by reference. Basic features of many of the Stirling machines described in the above referenced patents are also implemented in connection with the present invention.
The Stirling engine in accordance with the present invention has a so-called “modular” construction. The major components of the engine, comprising the drive case and cylinder block, are bolted together along mating surfaces. Piston rod seals for the pistons traverse this mating plane. A sliding rod seal can be used which is mounted either to the drive case or cylinder block. The rod seal controls leakage of the high pressure engine working gas at one end of the piston connecting rod to atmosphere.
In many past designs of Stirling engines, a large volume of the engine housing is exposed to the high working pressures of the working gas. In accordance with the engine of the present invention, the high pressure working fluid is confined to the extent possible to the opposing ends of the cylinder bores and the associated heat transfer devices and passageways. Thus the high pressure gas areas of the Stirling engine of this invention are analogous to that which is encountered in internal combustion engines, and therefore this Stirling engine can be thought of in a similar manner in terms of consideration for high pressure component failure. This benefit is achieved in the present invention by maintaining the drive case at a relatively low pressure which may be close to ambient pressure, while confining the high pressure working fluid within the cylinder block and the connected components including the cylinder extension, regenerator housing, and heater head.
The pistons of the engine are connected to cross heads by piston rods. The cross heads of the engine embrace the swashplate and convert the reciprocating movement of the piston connecting rods and pistons to rotation of the swashplate. The Stirling engine of this invention implements a pair of parallel guide rods mounted within the drive case for each cross head. The cross heads feature a pair of journals which receive the guide rods.
The combustion exhaust gases after passing through the heater head of the engine still contain useful heat. It is well known to use an air preheater to use this additional heat to heat incoming combustion air as a means of enhancing thermal efficiency. In accordance with this invention, an air preheater is described which provides a compact configuration with high thermal efficiency.
In the Stirling engine of the type according to the present invention employing four double acting cylinders, there are four discrete volumes of working gas which are isolated from one another (except by leakage across the pistons). In order to enable the engine to operate smoothly and with minimal force imbalances, the mean pressure of each of these four volumes need to be equalized. In accordance with this invention, this is achieved in part by connecting together the four volumes through small orifices. In addition, a system is provided for determining that the mean pressure in each cycle is within a predetermined range. Upon the occurrence of a component failure causing leakage, a significant imbalance could result which could have a destructive effect on the engine. The Stirling engine according to this invention features a pressure control system which unloads the engine upon the occurrence of such failure.
The Stirling engine in accordance with the present invention features a control valve component which, in part, provides the unloading feature mentioned previously. The control valve also provides one of the intended working gas leakage paths which forms part of the pressure balancing system in accordance with the present invention.
A critical component in the Stirling engine of the type described previously involves providing highly reliable seals between the high pressure displacer pistons and the low pressure drive case of the machine. Separating these two volumes is a piston rod seal assembly. Each piston connecting rod reciprocates through a piston rod seal which needs to reliably seal against the piston rod to maintain a low loss rate of working gas to the atmosphere. Absolute sealing of gas leakage through this area is likely not achievable. However, the piston rod seal assembly in accordance with the present invention provides low levels of leakage and reduces contamination of the working gas through “pumping” of lubricating oil in the drive case region.
Another critical design feature for enhancing efficiency of the Stirling engine comes from the design of the piston assembly. The displacer piston separates the hot and cold fluid spaces of the engine and reacts against gas pressures in these areas to deliver mechanical power. Thermal conduction losses across the piston between the hot and cold spaces need to be minimized to enhance efficiency. Moreover, a highly reliable sliding gas seal is required between the piston rings and the cylinder bore. In addition to constituting a thermal loss, such leakage across the piston seals further results in a net mass exchange of working gases between the individual cycle volumes of the Stirling engine. Significant differences in leakage across the piston seals can result in rapidly changing gas volumes in the cycle volumes. Although means are provided in accordance with this invention for reducing such imbalances, it is desirable to reduce the rate at which these imbalances occur.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.